1. Field of the Invention
The present invention relates to a power supply apparatus and, more specifically, to a power supply apparatus such as a DC-DC converter often used as a power source for a portable apparatus or the like.
2. Description of the Background Art
Conventionally, a power supply circuit referred to as a step up/step down type DC-DC converter such as shown in FIG. 3 has been used for portable audio equipments such as a portable CD and a headphone stereo or for personal communication service. Alternatively, voltage of a dry cell or a secondary cell is transformed to be suitable for each equipment by using a power supply circuit employing a coil with a center tap, not shown, in order to reduce number of cells and to make the equipment light and compact. When a portable equipment such as mentioned above is used while moving, battery voltage of about 1.5V to about 3.5V is stepped up/down to about 3.0V to be used as the power supply voltage. When a battery of an automobile or a DC adapter is available, a voltage of about 3.5V to about 12V input through a connector is stepped down to about 3.0V to be used as the power supply voltage.
The structure of the power supply circuit shown in FIG. 3 will be described. There are a power supply V1 having a voltage value higher than an output voltage V3 at an output terminal OUT, and a power supply V2 having a voltage value lower than the output voltage V3. Power supply V1 is applied to a down converter 50 for stepping down the voltage, and the output voltage of down converter 50 and power supply V2 are switched by a switch circuit S1. The voltage switched by switch circuit Si is output to output terminal OUT through an inductance element 51, a diode 52 and an output circuit 59. Inductance element 51 generates a counter electromotive force for stepping up, and diode 52 prevents reverse flow and decrease of the step up voltage. Output circuit 59 controls supply of output voltage V3 to a load, not shown, connected to output terminal OUT. A capacitor element 56 is connected between a node between diode 52 and output circuit 59 and a reference potential (GND). Capacitor element 56 charges current of counter electromotive force (hereinafter referred to as charging current), and stabilizes output voltage V3.
A transistor 55 is connected between a node between inductance element 51 and diode 52 and the reference potential. Transistor 55 is driven by a driving circuit 54. Driving circuit 54 contains an oscillating circuit and periodically renders transistor 55 conductive or non-conductive (hereinafter referred to as switching operation). Output voltage V3' on the anode side of diode 52 is applied to an output voltage detecting circuit 53.
Output voltage detecting circuit 53 detects whether or not output voltage V3' is at a prescribed level, and in response to detection of malfunction or failure of output voltage V3', applies a detection signal to driving circuit 54 and an output failure detecting circuit 57. Driving circuit 54 controls period of the switching operation of transistor 55 in accordance with the detection output from output voltage detecting circuit 53. Output failure detecting circuit 57 compares a voltage generated by rectifying and smoothing a clock signal of driving circuit 54 with the charging voltage, and detects malfunction of the circuit. In response to detection of malfunction of the circuit, output failure detecting circuit 57 controls output driving circuit 58 and controls conduction of output transistor of output circuit 59.
FIGS. 4A to 4C show waveforms at various portions of the control apparatus shown in FIG. 3. FIG. 4A shows voltage waveform of the collector of transistor 55, FIG. 4B shows current waveform flowing through inductance element 51 and FIG. 4C shows enlarged voltage waveform near the output voltage of output terminal OUT. In each figure, t1 represents period of switching operation of transistor 55, VF represents forward voltage of diode 52, and Vsat represents saturation voltage of the transistor in output circuit 59. For simplicity, voltage ripples and the like are omitted in waveforms other than FIG. 4C.
The operation of the power supply shown in FIG. 3 will be described with reference to FIGS. 4A to 4C. Transistor 55 performs switching operation in response to a clock signal from the oscillating circuit in driving circuit 54. When transistor 55 is rendered conductive and the collector voltage is at the reference potential as shown in FIG. 4A, current flowing through inductance element 51 gradually increases as shown in FIG. 4B. When transistor 55 is non-conductive and the collector voltage is (V3'+VF), the charging current flowing through inductance element 51 gradually decreases, as the counter electromotive force of inductance element 51 lowers. Thereafter, while there is sufficient charging current, transistor 55 is again rendered conductive, increasing the charging current.
By the repetition of such switching operation, capacitor element 56 is gradually charged and its charging voltage V3' increases, and the voltage of the power supply selected by switch circuit S1 is stepped up. When the charging voltage attains to a prescribed value or higher, the transistor of output circuit 59 is rendered conductive, and a prescribed output voltage V3 (=V3'-Vsat) shown in FIG. 4C is supplied to the load.
Under normal operating condition, the output voltage of output failure detecting circuit 57 attains to a high level, transistor 55 performs switching operation in response to a clock signal from driving circuit 54 and thus stepping up operation is performed. The transistor of output circuit 59 is rendered conductive through output driving circuit 58, and the stepped up at voltage is output. Further, driving circuit 54 controls transistor 55 such that when the charging voltage V3' is lower than the prescribed voltage value, switching frequency is increased so as to increase the speed of charging and when the charging voltage V3' attains to the prescribed value, the switching frequency is lowered, whereby the output voltage V3 is kept constant.
In case of a malfunction where output terminal OUT is short-circuited to the reference potential, output failure detecting circuit 57 sets the output voltage thereof to a low level, controls output driving circuit 58 so as to render the transistor of output circuit 59 non-conductive, and stops switching operation of driving circuit 54. Consequently, flow of short-circuit current from respective power supplies to output terminal OUT can be prevented, and the circuits in the power supply apparatus can be protected.
Output failure detecting circuit 57 compares a voltage generated by rectifying and smoothing the clock signal of driving circuit 54 with the charging voltage V3', and though stepping up operation is performed in the above described state of malfunction, the charging voltage V3' is not boosted.
Meanwhile, when the power supply V1 having the voltage higher than the output voltage V3 is to be stepped down and output, power supply V1 is selected by switch circuit S1. The voltage of power supply V1 is stepped down by down converter 50, capacitor 56 is charged by means of inductance element 51 and diode element 52, and the charging voltage is output to output terminal OUT through output circuit 59. At this time, output of the clock signal of driving circuit 54 is stopped, transistor 55 is kept non-conductive and the transistor of output circuit 59 is kept conductive.
Meanwhile, in the power supply apparatus shown in FIG. 3, there is much loss because of diode 52. Even when a shot key diode having as small a forward voltage as about 0.4V is used as diode 52, it is difficult to have conversion efficiency of the power supply apparatus not lower than 85%, and therefore it is difficult to elongate the time of operation of the portable equipment or the like.
Further, in order to reduce power consumption while the portable equipment is in a sleep state, it is necessary to provide output driving circuit 58 and output circuit 59 so as to prevent supply of voltage from output terminal OUT to the load. Further, since output failure detecting circuit 57 is used for protecting the power supply circuit from malfunction such as short-circuit of output terminal OUT to the reference potential, the circuit scale is considerably large and number of components used is large, resulting in increased area of the circuit board. From the foregoing, purchase and maintenance of components require much labor and cost, and outer size of the power supply apparatus is considerably large. Therefore, the outer size of the portable equipment using the power supply cannot be reduced, resulting in poor portability and high cost.
Meanwhile, the power supply apparatus using a coil with a center tap, not shown, is expensive and reduction in size of the equipment is difficult, since the coil is special and not versatile, and in addition, it is expensive.